1. Field of the Invention
This invention relates generally to a system and method for detecting a loss of cooling fluid from a thermal sub-system in a fuel cell system and, more particularly, to a system and a method for detecting a loss of cooling fluid from a thermal sub-system in a fuel cell system that uses current feedback from a high temperature pump that pumps the cooling fluid through the thermal sub-system.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack by serial coupling to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
As mentioned above, a fuel cell stack includes cooling fluid flow channels, typically in the stack bipolar plates, that receive a cooling fluid that maintains the operating temperature of the fuel cell at a desired level. The cooling fluid is pumped through the stack and an external coolant loop outside of the stack by a high temperature pump as part of a thermal sub-system, where a radiator typically cools the cooling fluid when it exits the stack. Temperature sensors are typically provided in the coolant loop external to the fuel cell stack to monitor the temperature of the cooling fluid as it exits and enters the stack to maintain a tight control of the stack temperature. The cooling fluid is typically a mixture of water and glycol that provides enhanced heat removal properties and reduces the freeze temperature of the cooling fluid.
If a component fails in the thermal sub-system, it is possible that the cooling fluid could leak from the thermal sub-system. If enough of the cooling fluid does leak from the thermal sub-system there may not be enough cooling fluid to reduce the temperature or maintain the desired temperature of the fuel cell stack, thus causing it to overheat, which could cause damage to various fuel cell system components, such as the fuel cells themselves. Therefore, it is known to employ devices and systems to detect cooling fluid leaks to protect the fuel cell system against overheating and potential component damage.
In one known leak detection design, a dedicated level sensor is employed to detect the level of the cooling fluid in an overflow tank or reservoir that holds the cooling fluid. However, there may be times when the level sensor indicates a low fluid level, but there may not be a significant leak, or no leak at all, and there may still be enough cooling fluid in the thermal sub-system to operate the stack. For example, if the vehicle turns a sharp corner, the fluid in the tank may fall below the level sensor, providing a false indication of a low cooling fluid. Further, for small cooling fluid losses, it may be desirable to only provide a warning indicator and not provide other mitigating actions, such as system shutdown.